1423721 - Inconsistent error message for "instanceof" right-hand-side.

Status: NEW

(Core :: JavaScript Engine, enhancement, P5)

Description

[Nicolas B. Pierron [:nbp]]

Currently, we have 2 different error messages for instanceof right-hand-side:
  TypeError: invalid 'instanceof' operand …
  TypeError: … is not a function

I suggest to replace both of these messages by a single one which says:
  TypeError: invalid right-hand-side operand of 'instanceof', expected a function, got …

js> 4 instanceof 5
TypeError: invalid 'instanceof' operand 5
js> [1,2,3] instanceof [];
TypeError: [] is not a function
js> [1,2,3] instanceof []
TypeError: [] is not a function
js> [1,2,3] instanceof {}
TypeError: ({}) is not a function
js> [1,2,3] instanceof ""
TypeError: invalid 'instanceof' operand ""
js> var x = [];
js> [1,2,3] instanceof x
TypeError: x is not a function

Comment 1

[Tooru Fujisawa [:arai]]

sometimes websites use the error message as condition, and changing the error message can result in breakage.
e.g. bug 1259822 caused bug 1490772, bug 1498257, and bug 1512401

while it's nice to improve the error message, we should prepare for breakage and fallback plan.